The present invention is directed to a spiral link belt, e.g. for the production of paper, which is composed of a multiplicity of mutually parallel spirals. The windings of successive spirals mesh with one another in the fashion of a zipper, and pintle wires are inserted through the channels formed by the intermeshing windings to flexibly connect the successive spirals.
Such spiral link belts are known from German Auslegeschrift No. 2,419,751, German Offenlegungsschrift No. 2,938,221, and EP-A-0,018,200. Normally the spirals consist of polyester resin monofilament. The spiral link belts are used as conveyor and filter belts in papermaking machines, predominantly in the drying section where they pass the formed but still moist paper sheet between individual heated drying cylinders thereby urging the sheet against the latter. The contact pressure improves heat transfer to the moist paper sheet and thus also improves the drying effect.
At the high operating speed common in modern papermaking machines from 800 to 1000 m/min, and even higher in some machines, considerable air currents develop within the drying section as air is entrained by the surface of rapidly rotating drying cylinders, and also by the drying screens and drying belts themselves. In the wedge-shaped gap where a drying spiral link belt, for example, runs onto a drying cylinder considerable superatmospheric pressure is built up by said air currents. Since the spiral link belts have a very loose structure, the air passes through the spiral link belts thereby lifting the paper sheet from the spiral link belt so that it starts to flutter and, in extreme cases, may even tear. In order to counteract this adverse effect attempts have been made to fill the interiors of the spirals with additional filament material. The filling consists of monofilament or multifilament threads; see German Offenlegungschriften No. 3,039,873 and 3,135,140 and French Pat. No. 2,494,318.
With the use of drying screens it sometimes happens that, during operation,, screens come in contact with the laterally provided stationary machine parts. As a result, the screen margin is destroyed very soon at the high operating speeds. While the two surfaces of the screen are smooth and consist of helix legs oriented predominantly in the longitudinal direction, the screen edges are rough along both sides. Each edge consists of severed helix wires and of the ends of the transverse pintle wires. This is the reason why the edges are destroyed in a very short time as soon as they chafe along the machine frame. The helices of the belt unwind from the edge inwardly. The same happens, though less rapidly, at the sensor rolls of the belt travel guide. Since the belts are very heavy, the sensors are moved under high pressure until the roll has been returned to the required position by the guide. This too, subjects the margins of the drying belt to very harsh treatment.
In order to overcome these difficulties the margins of the spiral link belts are pasted up. i.e. across a certain width they are filled with a curable synthetic resin composition, as shown in FIG. 7 of EP-A-0,052,350. However, after a long period of use at high temperatures and high humidity the synthetic resin composition grows brittle and crumbles. The synthetic resin composition cannont prevent the destruction of the margins of the drying belt upon contact with the stationary machine parts. As soon as the edge is damaged and opens up, the filler wires inserted into the spirals slide laterally out of the spiral link belt.
In order to remedy this shortcoming EP-A-0,052,350 proposes to insert specially shaped wire loops or wire spirals into the openings of the spirals from the sides. According to FIG. 2 of this publication, the edge is additionally reinforced by a longitudinally extending wire which is retained by the wire loops. The projecting ends of the inserted loops can also be interconnected by a longitudinally extending chain stitch.
This reinforcement of the edge has proved to be only partially helpful because, on account of the very small dimensions of the spaces between the helices the loops cannot be properly formed and inserted, and because the inserted loops are then thrown out of the spiral belt again at the high operating speed. Even if the margin is additionally pasted up with the synthetic resin composition, the loops remain in position only as long as the synthetic resin composition is still intact. The loops then form a reinforcement of the edge within the synthetic resin composition.
From German Offenlegunugsschrift No. 3,315,696, it is known to protect the screen margins by a protective member extending normal to the pintle wires and consisting, for example, of a steel wire or synthetic resin wire passed laterally along the edge and attached to the spiral belt by means of wires.
However, as the spiral link belt chafes against the frame of the machine, the laterally projecting wire loops can be destroyed leaving the protective member unsecured so that it travels along only as long as it is not caught by one of the stationary machine parts. The final tearing of the edge then renders the drying belt useless. In case the edge is additionally embedded into a synthetic resin composition, the resin composition nevertheless is in direct contact with the hot surface of the drying cylinders and is likewise subject to aging.